An important commercial application of optical scanners for encoded data is that of data processing in connection with supply, storage, and sale of manufactured goods. This invention was developed for application to automated checkstands, i.e., the so-called "point-to-sale" data processing for retail stores and the description will be given with respect to embodiments adapted therefor; however, as the description proceeds, it will be apparent that the invention is useful in a wide variety of optical scanning applications.
In the background of this invention is the adoption by the Uniform Grocery Products Code Council of a machine readable code symbol for the Grocery Industry Universal Products Code (UPC). This UPC symbol has been adopted as a standard for use in automated checkstands in grocery stores in the United States. This standard symbol is of bar code format, as will be described below. The practical application of this symbol depends upon the availability of code readers which have the capability of reading the symbol regardless of its orientation in the reading field, i.e., omnidirectional reading capability.
In the prior art there have been several proposals for achieving omnidirectional reading capability by optical scanners for code symbols of bar code format. A brief discussion of the prior art follows.
An optical code reader adapted for a code symbol of rectangular bar code format is set forth in the Myer U.S. Pat. No. 3,718,761. The system of this patent produces a scan pattern comprising a multiplicity of equiangularly displaced lines. This is obtained by using an optical beam having an elongated transverse cross-section (i.e., producing a line of light on a target) to scan the code symbol. The pattern is executed by scanning the line of light across the field by means of a multifaceted rotating mirror, rotating the line of light in the target plane by a small incremental angle and then scanning again in the new direction. If the angular increments are small enough, the line of light will intercept all of the elements of the code in at least one angular position so as to obtain a reading of the encoded data. This system has the advantage of being able to read a bar code in any orientation relative to the scan pattern even though the information code field is "undersquare," i.e., the height is less than the width, where the height is the dimension parallel to the bar code elements. The system of the Myer patent is disadvantageous in that is requires a complicated optical arrangement and stepwise motion which imposes an undesirably low limit on speed of operation in effecting readout of each code symbol.
Another optical code reader adapted for rectangular bar code symbols is set forth in the Munson U.S. Pat. No. 3,728,677. In the system of this patent a beam is caused to execute an X-scan pattern in a fixed field through which the code symbol is moved. The scan pattern includes two straight line traces in the plane of the scanning field with the traces intersecting at right angles. The traces are directed at an angle of 45.degree. relative to the direction of travel of the article bearing the code symbol. This scan pattern is produced by a multi-faceted. rotating mirror coacting with a pair of stationary reflectors which direct the beams from the laser onto the plane of the scanning field.
The Connell U.S. Pat. No. 3,818,444 also discloses an optical code reader for bar-code symbols. In the system of this patent a laser beam is passed through a beam splitter and each of the resulting beams is caused to execute a straight line scan, the scan motions occurring alternately and producing an X-scan pattern. Each of the resulting beams impinges upon a separate multifaceted scanner, each scanner having alternate light and dark facets. The scanners are rotated synchronously and have the light and dark facets out of phase so that only one beam is reflected at a time to produce a trace in the scanning field. Alternately, a single scanner may be used with a suitable beam splitting arrangement. The system of the Connell patent requires a beam splitter to produce a separate beam for each line or trace in the scan pattern.
A particular difficulty with the X-scan pattern of the aforementioned Munson patentand Connell patent is that it affords omnidirectional capability only if the information code field is at least square and for reliable reading it must be somewhat oversquare. Further, it imposes design limitations on the scanning field.